Stereoselective additions of chiral (E)-crotylsilanes to thionium ions: asymmetric synthesis of homoallylic thioethers.

Stereochemically well-defined homoallylic thioethers 3 are synthesized via Lewis acid promoted condensation reaction between chiral organosilane reagents 2 and in situ generated thionium ions. The stereochemical course of the reaction is consistent with earlier reports concerning crotylsilations of oxonium ions.